1. Field of the Invention
This invention relates to the recovery of fluid fuels from carbonaceous deposits and more particularly to an in-situ retort structure and a method of recovering fluid fuels from such retorts.
2. Description of the Prior Art
Immense potential sources of carbon-containing compounds suitable as fluid fuels exist in subsurface carbonaceous deposits of oil shale, coal, and heavy, highly viscous petroleum oils. The highly viscous petroleum oil deposits are frequently referred to as tar sands. Because the carbonaceous material in the deposits is either solid as in oil shale and coal or highly viscous as in tar sands, treatment of the carbonaceous deposit to make the carbon-containing compounds fluid is necessary to deliver them from the deposit to the surface. A method of treatment that has been used is to heat the deposit to a temperature at which fluid carbon-containing compounds are formed or the viscosity of heavy oils is drastically reduced. One method of heating the deposit is by in-situ combustion in which a portion of the carboniferous material in the deposit is burned in place by igniting the deposit and injecting air into the deposit to heat oil shale or tar sands to a temperature at which oils of low viscosity are produced or to produce combustible gaseous products from coal.
The very low permeability of oil shale to the flow of fluids makes it necessary to rubblize the shale to form an in-situ retort through which fluids for heating the shale to a temperature high enough to convert the kerogen to shale oil can be circulated. While sometimes coal and tar sands may be sufficiently permeable for an in-situ combustion process, rubblization of those deposits can be advantageous in reducing channeling through the deposits. One of the methods of forming an in-situ retort is described in U.S. Pat. No. 1,919,636 of Karrick. In the process described in that patent, a vertical central shaft is driven through the oil shale to provide the desired void space necessary for permeability and the oil shale is blasted from the walls of the shaft to fill the shaft with broken oil shale. Other mining procedures for forming a rubblized in-situ retort are described in U.S. Pat. No. 1,913,395 of Karrick, U.S. Pat. No. 2,481,051 of Uren, and U.S. Pat. No. 3,001,776 of Van Poolen. Those patents suggest using various mining techniques such as sublevel stoping, sublevel caving, block caving and shrinkage stoping to form the in-situ retort. In U.S. Pat. No. 3,917,347 of Janssen et al a method of constructing a retort having a cone-shaped ceiling is described.
Because of the great depth of the oil shale deposits, a substantial part of the oil shale must be left intact to support the overburden whether the shale is mined and retorted at the surface or is retorted in a modified in-situ process. If the oil shale should be mined for retorting at the surface and the full depth of the oil shale is mined, pillars having a large cross-sectional area would be necessary to support the overburden because of the great height of the pillars. In in-situ retorting processes, rubblized shale and the spent shale following retorting provide some lateral support for the pillars which would not exist in conventional mining operations. It is an object of this invention to provide an in-situ retort structure for the exploitation of a carbonaceous deposit which will provide support for the overburden and leave a minimum of unretorted carbonaceous material.